Thermionic valve amplifier



June; 17, 1941.

E. L. c. WHITE ETAL THERMIONIG VALVE AMPILIFIER Filed March 4, 1938 7 Lowfmpec/anc I Cob/e I l mus/woes v ERIC L. 0. WHITE JOHN HAgDW/CK BY 7%KAM ATTORNEY Patented June 17, 1941 John Hardwick, West Drayton, England, assignors to Electric & Musical Industries Limited, Hayes, Middlesex Great Britain England, a company of Application March 4, 1938, Serial No. 193,836 In Great Britain March 6, 1937 This invention relates to thermionic valve amplifiers and is particularly, but not exclusively, concerned with valve amplifiers used in television systems,

A single stage valve amplifier of the type commonly employed introduces phase reversal of the input signals, that is to say, the signal in the output circuit is 180 out of phase with that applied to the input of the valve. In the case of sound amplifiers, the phase reversal is not evident on reproduction, but in the case of picture signals as in television, phase reversal of the sig nals would convert a positive image into a negative image. It is of course easily possible to use another valve again to reverse the phase of the signals, but multiplication of components is of course uneconomical.

The object of the present invention is to provide an amplifier in which phase reversal between the input and output is avoided, and which does not therefore involve employing special means to correct for reversal of phase.

According to the present invention, an amplifier for amplifying signals from a low impedance source without reversal of phase between the input and output includes a valve having its grid connected effectively to earth and having an impedance connected in its anode lead from which the output is taken, the input signals being applied to the cathode oi the valve the conductance of which is such that the input impedance at the cathode with or without additional resistance matches the impedance of the source of signals. A resistance connected in the cathode lead of the valve may have a value which is large compared with the reciprocal of the mutual conductance of the valve. The grid of the valve may be connected to earth either directly or through a condenser, a direct current path between grid and cathode being provided in the latter case by a resistance. The input signals may be derived from a conductor fed from the cathode of a preceding valve both valves having their anodes connected to a common source of high tension supply and operating substantially in push-pull. In a particular application of the invention, the amplifier is used at the end of a television cable having the usual relatively low characteristic impedance, the low input impedance of the amplifler being particularly adapted to terminate the cable.

In order that the invention may be more clearly understood and readily carried into effect, two amplifying stages embodying the invention will now be described by way of example with reference to Figs. 1 and 2 of the accompanying drawing.

Referring to Fig. 1 of the drawing, the signals to be amplified without. phase reversal are applied to the input terminals I, 2 of an amplifying stage including a valve 3, the control grid of which is connected through a bias battery 4 to the earthed conductor 5. The terminal I is connected directly to the cathode of the valve between which and the earthed conductor is connected a resistance 6 the value of which "is large compared with l/g where g represents the mutual conductance of the valve 3. A battery 1 is connected in series with the resistance 6 and in such a sense as to oppose the voltage drop in the resistance 6 due to the valve current and thus to ensure that no potential difference will normally exist between the terminals I and 2.

It will be seen that as the input signals are applied directly to the cathode, the potential variations in the anode circuit will follow in phasethose of the input signals. A resistance It is connected in the anode lead and the output is taken from the terminals 8 and 9 connected respectively to the anode and the earthed conductcr. Since the grid which is effectively earthed is interposed between the input and output electrodes constituted respectively by the cathode and anode, there is little capacity coupling between these electrodes even in the case of the triode shown. Even for high frequencies it is not essential to employ a screened grid tetrode or a pentode although these types of valve may be used if desired.

The input impedance of the stage is very low, being practically l/g where g is the mutual conductance. This feature renders the amplifier particularly suitable for use at the end of a cable which is in any case required to be terminated by a resistance of the same order as 1/9 for average valves.

Again, in the case of a cable, the input end of which is connected across a resistance in the cathode lead of a valve operating in a similar manner to the valve shown in Fig. 1, it is an advantage to have a common anode supply for the valves at each end of the cable as their anode current changes are in opposition, the valves operating in push-pull, the current changes being made nearly or completely to cancel out, thus avoiding variations of H. T. voltage.

The arrangement of Fig. 1 shows the direct connection of grid to earth, but in Fig. 2 a circuit for alternating current coupling is shown. It will be seen in this case that a condenser I0 is interposed between the connection from the terminal I to the cathode of the valve and a second condenser H is inserted in the connection between the grid of the valve and the earthed conductor 5. A resistance [2 is inserted in order to provide a direct current path between the grid and cathode and is of sufficient value to provide adequate decoupling with condenser II. A further condenser I3 is inserted in the lead from the anode of the valve to the terminal 8. Resistances l4 and I5 are inserted in the cathode lead in order to carry the anode current, the grid being maintained negative with respect to the cathode as usual by means of the connection to the junction between the resistances l4 and I5. The resistance l4 may be of the usual few hundred ohms resistance required for a biasing resistance while the resistance I5 is large compared with /y-.

The insertion of condenser I0 renders necessary the provision of an inductance I1 and resistance I8 or their equivalent connected in series across the input terminals in order that the impedance presented at the input shall be maintained substantially constant.

It will be understood that the effective conductance of the valve may be augmented or diminished with the aid of resistance connected in series or parallel. Thus if the input impedance at the cathode of the valve differs from that of the source of signals by a small amount the valve conductance may be arranged to have a certain value which in conjunction with resistance added in series or in shunt with the input terminals or in the valve circuit will effect the desired matching.

We claim:

1. An amplifier for amplifying signals from a low impedance source without reversal of phase between the input and output including a valve having its grid connected effectively to earth and having an impedance connected in its anode lead from which the output is taken, the input signals being applied to the cathode of the valve the conductance of the valve being such that the input impedance at the cathode matches the impedance of the source of signals.

2. An amplifier for amplifying signals from a low impedance source without reversal of phase between the input and output including a valve having its grid connected effectively to earth and having an impedance connected in its anode lead from which the output is taken, the input signals being applied to the cathode of the valve the conductance of which is such that the input impedance at the cathode with additional resistance matches the impedance of the source of signals.

3. An amplifier according to claim 1 wherein there is connected in the cathode lead of the valve a resistance the value of which is large compared with the reciprocal of the mutual conductance of the valve.

4. An amplifier according to claim 2 wherein there is connected in the cathode lead of the valve a resistance the value of which is large compared with the reciprocal of the mutual conductance of the valve.

5. A television system including a feeder cable of low characteristic impedance connected to a single stage amplifier, the input and output signals of which are in the same phase, said amplifier comprising a valve having its grid connected eifectively to earth and having an impedance connected in its anode lead from which the output is taken, the input signals being applied to the cathode of the valve, the conductance of the valve being such that the input impedance at the cathode matches the characteristic impedance of the feeder cable.

6. A television system including a feeder cable of low characteristic impedance connected to a single stage amplifier, the input and output signals of which are in the same phase, said amplifier comprising a valve having its grid connected effectively to earth and having an impedance connected in its anode lead from which the output is taken, the input signals being applied to the cathode of the valve, the conductance of the valve being such that the input impedance at the cathode with additional resistance matches the characteristic impedance of the feeder cable.

7. An amplifier according to claim 1 wherein the connection of the grid of the valve effectively to earth is established through a condenser, a direct current path between the grid and cathode being provided by a resistance.

8. An amplifier according to claim 2 wherein the connection of the grid of the valve effectively to earth is established through a condenser, a direct current path between the grid and cathode being provided by a resistance.v

ERIC LAWRENCE CASLING WHITE. JOHN HARDWICK. 

